TOPEX/Poseidon (or simply T/P) is a cooperative Earth observation mission of the USA and France (NASA/JPL and CNES as partners) with the overall objective to provide high-accuracy global sea level (ocean height) measurements in coordinates relative to the center of the Earth. From this information, ocean circulation patterns can be mapped. The T/P data analysis helps to understand how the oceans interact with the atmosphere, and improve our ability to predict the global climate. NASA provided the satellite bus, five instruments, and is responsible for spacecraft operations. CNES furnished two of the spacecraft's instruments and the mission's Ariane launch vehicle. 1)2)

Background: Poseidon was originally a separate CNES mission in planning, but was later combined with TOPEX (agreement in 1987). TOPEX/Poseidon is the heart of the WOCE (World Ocean Circulation Experiment) program; it was also used for the campaigns of TOGA/COARE (Tropical Oceans and Global Atmosphere Experiment / Coupled Ocean Atmosphere Response Experiment). The T/P satellite is regarded the successor mission of the following US altimetry missions: GEOS-3 (Geodynamics Experimental Ocean Satellite-3, launch 1975), SEASAT (launch 1978) and GEOSAT (Geodetic Satellite, launch 1985). 3)4)5)6)

Figure 1: The Topex/Poseidon spacecraft illustration (zenith view)

Major mission objectives called for a dedicated altimetry mission. Combination of high altimetric precision and high orbital accuracy for the purpose of ocean topographic mapping. measurements of sea surfaces for modeling of global changes in ocean circulation and sea level (global panoramic maps of sea-surface topography); development of climate models for long-term forecasts (in the order of a season or longer). In particular, the requirements called for a precision orbit using three independent precision orbit determination systems.

Spacecraft:

The TOPEX/Poseidon spacecraft design is based on the Fairchild MMS (Multimission Modular Satellite) bus (with a heritage of Solar Maximum Mission and Landsat-4 and -5). The platform size is: 5.5 m in length, 6.6 m in height, and 2.8 m in width (span length of 11.5 m); the solar array size is 8.9 m x 3.3 m. The S/C consists of the MMS platform and the instrument module housing the sensors. The MMS in turn consists of the following elements: a command and data handling subsystem which includes the main on-board computer; the attitude determination and control subsystem, for maintaining the spacecraft attitude; and the electrical power subsystem, which contains the solar array and three batteries. The command and data handling subsystem houses three tape recorders for collecting engineering telemetry and instrument data. 7)

The S/C is three-axis stabilized (nadir pointing) using reaction wheels and magnetic torque rods as actuators. Attitude determination via Earth sensors, digital sun sensors, star cameras (ASTRA-1A, -1B), IRU (Inertial Reference Unit), and magnetometers. The S/C has a variety of communication antennas (steerable high-gain antenna dish and two omni antennas) to link the mission with TDRS (Tracking Data Relay Satellite), with the DORIS tracking system, and with GPS. The nominal T/P design life is 3 years plus 2 years for an extended mission.

The satellite mass is 2388 kg (launch mass) and 2169 kg of dry mass, solar power = 3.4 kW (BOL); the three batteries have a capacity of 50 Ah. A hydrazine propellant system is being used for orbit maintenance. Mission operations are being conducted at POCC (Project Operations Control Center) at JPL, Pasadena, CA. 8)9)

3) An experimental GPS receiver, GPSDR, is being used to track the satellite

4) Use of the TOPEX/Poseidon radar altimeter data.

While the SLR and DORIS measurement techniques are limited in coverage due to the nature of a ground-based system, the onboard GPS provides continuous tracking of the satellite. Furthermore, the RF communication via TDRS offers still another tracking method of the spacecraft.

A posteriori analysis of the tracking information from the four sources has demonstrated a satellite position determination capability in the < 5 cm altitude range and the calculation of ocean height within 1-2 cm every 10 days. The multi-source tracking strategy of T/P is a first in high-performance POD analysis. Many other projects followed this hybrid POD approach.

Measurement

Required accuracy

Achieved accuracy

Altimeter

4.0 cm

3.2 cm

Satellite position

12.8 cm

2.8 cm

SSH (Sea Surface Height)

13.4 cm

4.3 cm

Table 2: Measurement accuracy of TOPEX/Poseidon

Measurement calibration at start of mission (CALVAL): CNES and NASA each established a verification site along the ground track of the satellite. The CNES verification site is on Lampedusa Island (Italy) in the Mediterranean Sea. The NASA site is on Platform Harvest, an offshore oil rig off the coast of California. In-situ (on-site) verification requires a measure of the altimeter-to-ocean distance that is independent of the measurement made by the satellite. To obtain this independent value, in situ sea level measurements must be accurately tied to the reference frame of the satellite. This was accomplished by combining a number of measurements obtained through different techniques.

- Use of SLR techniques from each verification site

- Vertical measurements are being made from a GPS antenna at the verification site to the on-site sea-level-measurement instruments.

From these data, an estimate is made of the system bias, usually expressed in terms of altimeter bias, which is the difference between the altimeter-to-ocean distance derived from ground-based corroborating instruments and the distance measured by the TOPEX/Poseidon altimeters. 10)11)12)

One of the achievements of the GPS system on T/P was to permit the calculation of orbits using a reduced-dynamic technique. The intercomparison of the reduced-dynamic orbits available from GPS, and the dynamic orbits available from SLR and DORIS provided a valuable means to identify and eventually reduce force model errors, such as geographically correlated orbit error from gravity field mis-modeling, and stationary errors from dynamic ocean tide mis-modeling. 13)

The precise orbits that are used for the geophysical data records (GDRs) rely on the processing and analysis of SLR, DORIS, and altimeter crossover data. T/P achieved routine and reliable delivery of orbits with a radial accuracy of 2.5 cm. This success was due to the precision of the complementary tracking systems, SLR, DORIS, and GPS that were carried by the spacecraft.

Figure 5: Illustration of the TOPEX7Poseidon spacecraft components (image credit: University of Texas)

Mission status: TOPEX/Poseidon operated for 13 years from 1992 to 2005

• August 10, 2017: Today marks the 25th anniversary of the launch of a revolutionary ocean research vessel — a space "ship." As the NASA/CNES TOPEX/Poseidon satellite ascended into orbit, it ushered in a new era of oceanography with the first highly accurate, global measurements of sea levels. That mission and its three successors, all named Jason, have continuously mapped global ocean currents and tides; opened our eyes to the global reach of El Niño and other climate events; created a quarter-century-long, extraordinarily precise record of global and regional sea level rise; and enabled improved forecasts of extreme weather events such as hurricanes, floods and droughts. 14)

Figure 6: Changes in sea level height from 1993 to 2017 compared with a long-term mean of the data. Blue and purple are lower than the mean; red, yellow and white are higher (image credit: NASA/JPL-Caltech)

- In 1992, when TOPEX/Poseidon launched, no one foresaw that its record of precision ocean height measurements would continue through three decades and four spacecraft. In fact, many oceanographers at the time weren't convinced that TOPEX/Poseidon's sensors would be accurate enough to reveal the signal of sea level rise out of the noise of waves, tides and other changes. But the radar altimeter and radiometer measurement system outperformed expectations from the start. In 25 years of continuous operation, TOPEX/Poseidon and its successors have recorded 7 cm of global average sea level rise.

- Our planet's oceans are too vast and complex to be fully measured by any single satellite, or even by any single nation. TOPEX/Poseidon and its successor Jason satellite missions are shining examples of the power of a sustained, long-term international partnership, led by the U.S. and French space agencies, NASA and CNES. For nearly three decades, NASA and CNES scientists and engineers have pooled their expertise, talents and insights to design and construct an integrated spaceborne measurement system far more powerful than the sum of its parts. NASA and CNES have worked together, applying advanced technology to collect measurements of remarkable precision and accuracy, and then making those measurements freely and openly available. With this effort, they have provided humanity with unprecedented views of the global oceans, how they change on time scales of days to decades, and how the oceans influence — and respond to — weather and climate.

- "For more than a generation, NASA and CNES scientists and engineers have collaborated to make exquisitely accurate measurements of the ocean surface from space, providing insights into the workings and interactions of our planet's two great fluid systems, the oceans and the atmosphere," said Michael Freilich, director of NASA's Earth Science Division in Washington.

• The TOPEX/Poseidon oceanography satellite, launched Aug. 10, 1992, ceased operations on Oct. 9, 2005, after nearly 62,000 orbits of Earth in 13 years of operations. The spacecraft's pitch reaction wheel stalled causing the loss of 3-axis stabilization (the ability to maneuver), hence the end of the mission. The T/P mission was decommissioned on January 18, 2006. The mission had far exceeded expectations in terms of both mission duration (design life of 5 years) and measurement system performance. 15)16)17)

- Everybody, and in particular anyone who has been involved from the very beginning, is now left with the feeling that they were part of a great adventure, from a technical and above all a human perspective. During its thirteen years of operation, Topex/Poseidon witnessed many developments and innovations and even some media success. As for its overall success, the mission will be remembered both for the expected applications and the additional ones which led to greater interest in this type of satellite data. 18)

- TOPEX/Poseidon revolutionized the study of Earth's oceans, providing the first continuous, global coverage of ocean surface topography. Its data made a huge difference in our understanding of the oceans and their affect on global climatic conditions. The data helped in hurricane and El Nino/La Nina forecasting, ocean and climate research, ship routing, offshore industries, fisheries management, marine mammals' research, modernizing global tide models and ocean debris tracking. The TOPEX/Poseidon mission attracted users around the world, including more than 600 scientists in 54 countries.

- The mission's most important achievement was to determine the patterns of ocean circulation — how heat stored in the ocean moves from one place to another. Since the ocean holds most of the Earth's heat from the sun, ocean circulation is a driving force of climate. 19)

- Another of the mission's major accomplishments was to map global tides for the first time. Tides are important for navigation, they have a big role in biological activity, and they are the major source of mixing in the ocean.

• 2002: From mid-August onwards, during its tenth year, TOPEX/Poseidon was gradually transferred to a new orbit, a maneuver which was completed in mid-September 2002. The new ground track of the orbit was halfway between its old tracks (henceforth those of Jason-1). This tandem phase enabled measurements to be provided from two inter-calibrated, precise altimeters working together on similar orbits 158 km apart at the equator (Ref. 20).

- On March 1, 2002, the Envisat Earth observing satellite was launched. Onboard, among the ten instruments, an altimeter. An aborted El Niño phenomenon was observed in 2002. Models forecasted a 'moderate' phenomenon, which in fact disappeared completely before reaching the South American coast. This kind of unexpected event highlighted the importance of long-term observations in order to better understand such phenomena and their variations, so as to model them correctly.

• Starting with the launch of Jason-1 (Dec. 7, 2001), a closely-spaced tandem mission was flown with the T/P and the Jason-1 satellites, both following an exact repeat orbit of 1336 km altitude. This orbital configuration was changed (Aug. 15, 2002) when the orbit of T/P was slowly shifted to be midway between its former tracks (with only 1 minute separation between the two spacecraft). The new configuration was reached on Sept. 20, 2002. This orbital configuration provided interleaved ground tracks with a 1.4º longitude spacing from the Jason‐1 tracks - permitting frequent cross‐calibrations which resulted in ocean topography data with unprecedented accuracy (better than either S/C could attain by itself). The inter-satellite calibration has determined the relative bias to an accuracy of 1.6 mm. Results from the merging of T/P and Jason-1 data have confirmed the potential of an optimized two-satellite configuration for mesoscale variability studies. The tandem mission of Jason-1 and TOPEX/Poseidon lasted until Oct. 2005, when the TOPEX/Poseidon mission ended due to a failure in a pitch reaction wheel.

• From February to April 1997, TOPEX/Poseidon satellite data pinpointed a large eastward-moving swelling of waters in the central Pacific. This positive sea-level anomaly, over 10 cm and increasing as the months went by, peaked near the coast. In July 1997 (Figure 7), the signature was plain and south-east Asia was hit by severe drought. The anomaly, in red on the map, covered an area 1.5 times that of the United States. It corresponded to "extra" warm waters. 20)

- TOPEX/Poseidon tracked the development of El Niño, highlighting a maximum anomaly of more than 20 cm in the northern winter. By June 1998, surface height was returning to normal. In July 1998, TOPEX/Poseidon revealed favorable conditions for La Niña, which developed in 1999. In 2000, ocean once more returned to normal.

• The onboard DORIS receiver experienced a malfunction on Nov. 1, 2004. Since then, the T/P orbits were computed using a combination of SLR and altimeter crossover data.

• The GPS Monarch receiver (GPSDR) could only track GPS satellites through six channels (GPSDR was not able to receive P-code measurements) and operated in dual-frequency mode only while A/S (antispoofing) was turned off. This limited the use of the GPS data for precise orbit applications to approximately the first 16 months in orbit or through January 1994.

• The first altimetry measurements from TOPEX/Poseidon actually came from the prototype Poseidon-1 instrument, on 21 August 1992, as it was difficult to program the main Topex instrument until the satellite reached its permanent orbit (Ref. 17).

Sensor complement: (NRA, TMR, GPSDR, SSALT, LRA, DORIS)

NRA (NASA Radar Altimeter)

NRA is of GEOS-3, Seasat, and Geosat heritage. NRA uses a linear FM chirp pulse centered at 13.6 GHz (Ku-band) and at 5.3 GHz (C-band), the dual-frequency design is used to correct for ionospheric path delays (first spaceborne dual-frequency altimeter). The range difference measured at these two frequencies provided a first-order correction for the influence of the ionosphere. TheNRA instrument consisted of a signal processor, an RF section, and an antenna assembly. 21)22)

NRA was designed and built by JHU/APL, and managed by GSFC. It is the prime sensor for the measurement of sea surface heights, wave heights, and surface wind speed. The Ku-band and C-band bandwidth is 320 MHz, with C-band selectable at 100 MHz; the antenna control pointing accuracy is 0.14º (1 σ).NRA mass = 230 kg, power = 237 W, altitude measurement accuracy of 2.4 cm. Simultaneous measurements at both frequencies so that ionospheric range delay can be directly estimated from the two measurements.

TMR is of Nimbus-7 heritage. Operation at 18, 21, and 37 GHz to measure the total water vapor content along the altimeter pulse path to correct for water-vapor-induced range delay. The uncertainty in the altimeter range measurement made by such a system under normal ocean conditions is expected to be less than 5 cm at 7 km spatial resolution (3 cm at 100 km resolution). Mass = 50 kg, power = 25 W.

TMR used a 79 cm diameter nadir-pointed offset paraboloid reflecting antenna to measure atmospheric emissivity. It also used a cold blackbody calibration with a cold sky horns looking at right angles to the sun line. The instrument consisted of RF/data modules, power supply modules, cold horns, and multifrequency feed horn. The RF/data modules and power supply modules were located directly beneath the feed horn. The instrument was thermally controlled by louvers and replacement heaters. The footprint of the 21 GHz channel was about 35 km. Measurements of the columnar water vapor along the satellite ground track had an accuracy of 0.2 gm/ cm2 over a range of 0.2 to 6.0 gm/cm2. TMR data provided corrections to altimeter height data for the effects of atmospheric water vapor to an uncertainty of 1.2 cm.

GPSDR (GPS Demonstration Receiver)

The GPSDR system (Monarch) is for direct position measurement, JPL (manufacturer: Motorola); Demonstration of GPS differential ranging as an experiment. The GPS receiver measures incoming signals from the GPS satellites and uses in addition ITRF (International Terrestrial Reference Frame, i.e. a set of reference ground stations) measurements for DGPS results. GPSDR operates at 1227.6 MHz and at 1575.4 MHz. Mass = 28 kg, power = 29 W, altitude accuracy < 10 cm. The TRANET orbit determination (with the ground network) was dropped in favor of DORIS and laser approaches.

SSALT (Single-Frequency Solid-State Altimeter)

SSALT is also referred to as Poseidon-1. The single instrument frequency is 13.65 GHz (Ku-band). SSALT is an experimental altimeter (of CNES, built by Alcatel Alenia Space, formerly Alcatel Espace, Toulouse) to demonstrate the concept of low-power, low-mass, low data rate (1/7 the rate ofNRA due to extensive on-board processing) and low-cost altimeter for future Earth observing missions. Ionospheric range correction is provided by a model that makes use of simultaneous DORIS measurements. Measurement accuracy = 2.5 cm. The Poseidon-1 instrument mass = 24 kg, power = 49W (the experimental Poseidon-1 altimeter was 4 times as light and consumed 5 times less power than the NRA). 23)

Poseidon-1 consists of two packages: the processing and control unit (PCU), and the radio frequency unit (RFU). The Poseidon altimeter sends a pulse generated by a surface acoustic wave generator with a 300 MHz bandwidth and 900 MHz frequency to the sea surface. The pulse is frequency converted to 13.65 GHz after generation. The pulses are amplified before transmission. The received pulses are amplified and an FFT analyzer computed the power spectrum of the signal. The signal is then sent through a microprocessor which performs the waveform computations. The output data is related to the sea state and wind speed.

The TOPEX/Poseidon LRA, was a set of 192 quartz corner-cubes mounted in two concentric rings around the altimeter antenna. The array is a completely passive unit which reflects laser beams originating from one of about thirty ground-based laser tracking stations positioned around the Earth. By measuring the length of time the laser beam takes to travel to the spacecraft and back, scientists were able to calculate TOPEX/Poseidon's orbital radial position to within 3 cm. 24)25)

The DORIS tracking system is a new development by the French organizations: CNES (Centre National D'Etudes Spatiales), GRGS (Groupe de Recherches de Géodésie Spatiale), and IGN (Institut Géographique National). DORIS is a one-way microwave tracking system for the determination of precise orbits (3-10 cm radial distance and 0.3 mm/s in range rate accuracy). The concept is based on a ground segment (of globally positioned tracking stations) and a space segment (i.e. DORIS as a passenger payload in a satellite consisting of a receiver, an ultra-stable oscillator, and an antenna). There is also a control center as part of the ground segment, located at CNES. 26)27)

The onboard receiver measures the Doppler shift of `uplink beacons' in two frequencies (f1 =2036.25MHz, f2 = 401.25 MHz), which are transmitted continuously by the DORIS ground network of stations (50). One measurement is used to determine the radial velocity between spacecraft and beacon, the other to eliminate errors due to ionospheric propagation delays. Only one beacon can be received by the space segment at any time (Note: a second generation receiver starting with DORIS/ENVISAT will be able to treat simultaneously two beacons). Orbit determinations with a precision of <5 cm on the radial distance component are achieved on the TOPEX/Poseidon mission.

- a beacon installation and management center, managed by IGN. A network of Orbit-Determination Beacons (ODBs) is positioned throughout the world.

- Precision orbit determination computations performed by CNES (Earth's gravitational field computation on the basis of DORIS data by GRGS).

An ODB comprises two transmitters (one operating at 401.25 MHz, the other at 2036.25 MHz), an ultrastable oscillator, and a microprocessor performing the necessary control and management functions, transmission of timing, housekeeping, and failure diagnosis. An ODB also includes an antenna and three meteorological sensors (atmospheric pressure, air temperature, and relative humidity); these parameters are needed for atmospheric propagation delays. An ODB message carries meteorological data, the beacon ID, and information concerning the beacon operating status. The complete message lasts 0.8 seconds and is repeated once every 10 seconds.

A second class of beacons is termed Ground Location Beacons (GLBs). These are at positions that are either unknown or not known to sufficient accuracy. GLBs use the results of high-precision orbit determination as input for the precise determination of ground positions. GLBs are functionally identical to ODBs. Each GLB transmits independently of all others for 10 seconds, once, twice, or three times every minute, but only while the satellite is in range. These beacons are used for precise positioning applications. Since October 1994 DORIS measurements are included in IERS (International Earth Rotation Service).
The master beacon (MB) is the link between DCC and the on-board package. On each pass the DCC transmits instructions for on-board programming and time-tagging information. DORIS time is based on the on-board receiver clock which is related to UTC using master beacon measurements.

The geophysical data produced by the TOPEX/Poseidon mission are being made accessible to the international scientific community through the US data center at JPL called PODAAC (Physical Oceanographic Distributed Active Archive Center), and through the French data center, AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data). The T/P data products include: 28)

• Sea surface topography

• Significant wave height

• Surface wind speed

• Ocean tides

• Vertically integrated atmospheric water vapor

• Vertically integrated ionospheric electron content

Some T/P Results

TOPEX/Poseidon data has revolutionized the way the global ocean is studied. For the first time, the seasonal cycle and other temporal variabilities of the ocean have been determined globally with high accuracy, yielding fundamentally important information for testing ocean circulation models. Major observations were made using TOPEX/Poseidon data on: 29)30)

• Oceanic circulation including details on the movement of Rossby and Kelvin waves

• Oceanic and coastal tides

• El Niño, La Niña, and the Pacific Decadal Oscillation

• El Niño-like circulation in the Atlantic Ocean

• Oceanic seasons in the Mediterranean

• Ocean floor topography from surface data used to refine the geoid model

The primary TOPEX/Poseidon mission has actually been completed as of August 1995. The data set from this prime mission has provided oceanographers with the first global data set on the Earth's oceans.

The Jason-1 mission of NASA/CNES (launch Dec. 7, 2001) is the follow-on to T/P. The Jason-1 operational orbit follows an exact repeat ground track every 127 revolutions in ten days with the same characteristics as those of T/P (identical orbital tracks (about a minute apart) to perform cross calibration). In this tandem setup (completed Sept. 16, 2002), Jason-1 is located one minute ahead of T/P. Both missions are providing high-resolution topography measurement data sets. 31)

2) "Predicted Topex Positioning Accuracy with Differential GPS Techniques," presented at, and published in the `Proceedings of the first International Symposium on Precise Orbit Positioning with GPS' April 15, 1985

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.